Method for reduction of aliasing introduced by spectral envelope adjustment in real-valued filterbanks

ABSTRACT

The present invention proposes a new method for improving the performance of a real-valued filterbank based spectral envelope adjuster. By adaptively locking the gain values for adjacent channels dependent on the sign of the channels, as defined in the application, reduced aliasing is achieved. Furthermore, the grouping of the channels during gain-calculation, gives an improved energy estimate of the real valued subband signals in the filterbank.

TECHNICAL FIELD

[0001] The present invention relates to systems comprising spectralenvelope adjustment of audio signals using a real-valued subbandfilterbank. It reduces the aliasing introduced when using a real-valuedsubband filterbank for spectral envelope adjustment. It also enables anaccurate energy calculation for sinusoidal components in a real-valuedsubband filterbank.

BACKGROUND OF THE INVENTION

[0002] It has been shown in PCT/SE02/00626 “Aliasing reduction usingcomplex exponential modulated filterbanks”, that a complex-exponentialmodulated filterbank is an excellent tool for spectral envelopeadjustment audio signals. In such a procedure the spectral envelope ofthe signal is represented by energy-values corresponding to certainfilterbank channels. By estimating the current energy in those channels,the corresponding subband samples can be modified to have the desiredenergy, and hence the spectral envelope is adjusted. If restraints oncomputational complexity prevents the usage of a complex exponentialmodulated filterbank, and only allows for a cosine modulated(real-valued) implementation, severe aliasing is obtained when thefilterbank is used for spectral envelope adjustment. This isparticularly obvious for audio signals with a strong tonal structure,where the aliasing components will cause intermodulation with theoriginal spectral components. The present invention offers a solution tothis by putting restraints on the gain-values as a function of frequencyin a signal dependent manner.

SUMMARY OF THE INVENTION

[0003] It is the object of the present invention to provide an improvedtechnique for spectral envelope adjustment.

[0004] In accordance with a first aspect of the invention, this objectis achieved by an apparatus for spectral envelope adjustment of asignal, comprising: means for providing a plurality of subband signals,a subband signal having associated therewith a channel number kindicating a frequency range covered by the subband signal, the subbandsignal originating from a channel filter having the channel number k inan analysis filterbank having a plurality of channel filters, whereinthe channel filter having the channel number k has a channel responsewhich is overlapped with a channel response of an adjacent channelfilter having a channel number k−1 in an overlapping range; means forexamining the subband signal having associated therewith the channelnumber k and for examining an adjacent subband signal having associatedtherewith the channel number k−1 to determine, whether the subbandsignal and the adjacent subband signal have aliasing generating signalcomponents in the overlapping range; means for calculating a first gainadjustment value and a second gain adjustment value for the subbandsignal and the adjacent subband signal in response to a positive resultof the means for examining, wherein the means for calculating isoperative to determine the first gain adjustment value and the secondgain adjustment value dependent on each other; and means for gainadjusting the subband signal and the adjacent subband signal using thefirst and the second gain adjusting values or for outputting the firstand the second gain adjustment values for transmission or storing.

[0005] In accordance with a second aspect of the invention, this objectis achieved by a method of spectral envelope adjustment of a signal,comprising: providing a plurality of subband signals, a subband signalhaving associated therewith a channel number k indicating the frequencyrange covered by the subband signal, the subband signal originating froma channel filter having the channel number k in an analysis filterbankhaving a plurality of channel filters, wherein the channel filter havingthe channel number k has a channel response which is overlapped with achannel response of an adjacent channel filter having a channel numberk−1 in an overlapping range; examining the subband signal havingassociated therewith the channel number k and for examining an adjacentsubband signal having associated therewith the channel number k−1 todetermine, whether the subband signal and the adjacent subband signalhave aliasing generating signal components in the overlapping range;calculating a first gain adjustment value and a second gain adjustmentvalue for the subband signal and the adjacent subband signal in responseto a positive result of the means for examining, wherein the means forcalculating is operative to determine the first gain adjustment valueand the second gain adjustment value dependent on each other; and gainadjusting the subband signal and the adjacent subband signal using thefirst and the second gain adjusting values or outputting the first andthe second gain adjustment values for transmission or storing.

[0006] In accordance with a third aspect of the invention, this objectis achieved by a computer program having a program code for performingthe above method, when the computer program runs on a computer.

[0007] In accordance with a fourth aspect of the invention, this objectis achieved by a method for spectral envelope adjustment of a signal,using a filterbank where the filterbank comprises a real valued analysispart and a real valued synthesis part or where said filterbank comprisesa complex analysis part and a real valued synthesis part, where a lower,in frequency, channel and an adjacent higher, in frequency, channel aremodified using the same gain value, if the lower channel has a positivesign and the higher channel has a negative sign, so that a relationbetween subband samples of the lower channel and subband samples of thehigher channel is maintained.

[0008] The present invention relates to the problem of intermodulationintroduced by aliasing in a real-valued filterbank used for spectralenvelope adjustment. The present invention analyses the input signal anduses the obtained information to restrain the envelope adjustmentcapabilities of the filterbank by grouping gain-values of adjacentchannel in an order determined by the spectral characteristic of thesignal at a given time. For a real-valued filterbank e.g. a pseudo-QMFwhere transition bands overlap with closest neighbour only, it can beshown that due to aliasing cancellation properties the aliasing is keptbelow the stop-band level of the prototype filter. If the prototypefilter is designed with a sufficient aliasing suppression the filterbankis of perfect reconstruction type from a perceptual point of view,although this is not the case in a strict mathematical sense. However,if the channel gain of adjacent channels are altered between analysisand synthesis, the aliasing cancellation properties are violated, andaliasing components will appear audible in the output signal. Byperforming a low-order linear prediction on the subband samples of thefilterbank channels, it is possible to asses, by observing theproperties of the LPC polynomial, where in a filterbank channel a strongtonal component is present. Hence it is possible to assess whichadjacent channels that must not have independent gain-values in order toavoid a strong aliasing component from the tonal component present inthe channel.

[0009] The present invention comprises the following features:

[0010] Analysing means of the subband channels to asses where in asubband channel a strong tonal component is present;

[0011] Analysing by means of a low-order linear predictor in everysubband channel;

[0012] Gain grouping decision based on the location of the zeros of theLPC polynomial;

[0013] Accurate energy calculation for a real-valued implementation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will now be described by way ofillustrative examples, not limiting the scope or spirit of theinvention, with reference to the accompanying drawings, in which:

[0015]FIG. 1 illustrates a frequency analysis of the frequency rangecovered by channel 15 to 24 of an M channel subband filterbank, of anoriginal signal containing multiple sinusoidal components. The frequencyresolution of the displayed analysis is intentionally higher than thefrequency resolution of the used filterbanks in order to display wherein a filterbank channel the sinusoidal is present;

[0016]FIG. 2 illustrates a gain vector containing the gain values to beapplied to the subband channels 15-24 of the original signal.

[0017]FIG. 3 illustrates the output from the above gain adjustment in areal-valued implementation without the present invention;

[0018]FIG. 4 illustrates the output from the above gain adjustment in acomplex-valued implementation;

[0019]FIG. 5 illustrates in which half of every channel a sinusoidalcomponent is present;

[0020]FIG. 6 illustrates the preferred channel grouping according to thepresent invention;

[0021]FIG. 7 illustrates the output from the above gain adjustment in areal-valued implementation with the present invention;

[0022]FIG. 8 illustrates a block diagram of the inventive apparatus;

[0023]FIG. 9 illustrates combinations of analysis and synthesisfilterbanks for which the invention can be advantageously used.

[0024]FIG. 10 illustrates a block diagram of the means for examiningfrom FIG. 8 in accordance with the preferred embodiment; and

[0025]FIG. 11 illustrates a block diagram of the means for gainadjusting from FIG. 8 in accordance with the preferred embodiment of thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] The below-described embodiments are merely illustrative for theprinciples of the present invention for improvement of a spectralenvelope adjuster based on a real-valued filterbank. It is understoodthat modifications and variations of the arrangements and the detailsdescribed herein will be apparent to others skilled in the art. It isthe intent, therefore, to be limited only by the scope of the impendingpatent claims and not by the specific details presented by way ofdescription and explanation of the embodiments herein.

[0027] In the following description a real-valued pseudo-QMF is usedcomprising a real-valued analysis as well as a real valued synthesis. Itshould be understood however, that the aliasing problem addressed by thepresent invention also appears for systems with a complex analysis and areal-valued synthesis, as well as any other cosine-modulated filterbankapart from the pseudo-QMF used in this description. The presentinvention is applicable for such systems as well. In a pseudo-QMF everychannel essentially only overlaps its adjacent neighbour in frequency.The frequency-response of the channels is shown in the subsequentfigures by the dashed lines. This is only for illustrative purposes toindicate the overlapping of the channels, and should not be interpretedas the actual channel response given by the prototype filter. In FIG. 1the frequency analysis of an original signal is displayed. The figureonly displays the frequency range covered by to of the M channelfilterbank. In the following description the designated channel numbersare derived from their low cross-over frequency, hence channel 16 coversthe frequency range to excluded the overlap with its neighbours. If nomodification is done to the subband samples between analysis andsynthesis the aliasing will be limited by the properties of theprototype filter. If the subband samples for adjacent channels aremodified according to a gain vector, as displayed in FIG. 2, withindependent gain values for every channel the aliasing cancellationproperties are lost. Hence an aliasing component will show up in theoutput signal mirrored around the cross-over region of the filterbankchannels, as displayed in FIG. 3. This is not true for an compleximplementation as outlined in PCT/SE02/00626 where the output, asdisplayed in FIG. 4, would not suffer from disturbing aliasingcomponents. In order to avoid the aliasing components that causes severeintermodulation distortion in the output, the present invention teachesthat two adjacent channels that share a sinusoidal component as e.g.channel 18 and 19 in FIG. 1, must be modified similarly, i.e. the gainfactor applied to the two channels must be identical. This is hereafterreferred to as a coupled gain for these channels. This of course impliesthat the frequency resolution of the envelope adjuster is sacrificed, inorder to reduce the aliasing. However, given a sufficient number ofchannels, the loss in frequency resolution is a small price to pay forthe absence of severe intermodulation distortion.

[0028] In order to assess which channels should have coupledgain-factors, the present invention teaches the usage of in-band linearprediction. If a low order linear prediction is used, e.g. a secondorder LPC, this frequency analysis tool is able to resolve onesinusoidal component in every channel. By observing the sign of thefirst predictor polynomial coefficient it is easy to determine if thesinusoidal component is situated in the upper or lower half of thefrequency range of the subband channel.

[0029] A second order prediction polynomial

A(z)=1−α₁ z ⁻¹−α₂ z ⁻²   (1)

[0030] is obtained by linear prediction using the autocorrelation methodor the covariance method for every channel in the QMF filterbank thatwill be affected by the spectral envelope adjustment. The sign of theQMF-bank channel is defined according to: $\begin{matrix}{{{sign}(k)} = \left\{ {\begin{matrix}\left( {- 1} \right)^{k} & {{{if}\quad \alpha_{1}} < 0} \\\left( {- 1} \right)^{k + 1} & {{{if}\quad \alpha_{1}} \geq 0}\end{matrix},{0 < k < M},} \right.} & (2)\end{matrix}$

[0031] where k is the channel number, M is the number of channels, andwhere the frequency inversion of every other QMF channel is taken intoaccount. Hence, it is possible for every channel to asses where a strongtonal component is situated, and thus grouping the channels togetherthat share a strong sinusoidal component. In FIG. 5 the sign of eachchannel is indicated and hence in which half of the subband channel thesinusoidal is situated, where +1 indicates the upper half and −1indicates the lower half. The invention teaches that in order to avoidthe aliasing components the subband channel gain factors should begrouped for the channels where channel k has a negative sign and channelk−1 has a positive sign. Accordingly the channel signs as illustrated byFIG. 5 gives the required grouping according to FIG. 6, where channel 16and 17 are grouped, 18 and 19 are grouped, 21 and 22 are grouped, andchannel 23 and 24 are grouped. This means that the gain values g_(k)(m)for the grouped channels k and k−1 are calculated together, rather thanseparately, according to: $\begin{matrix}{{{g_{k}(m)} = {{g_{k - 1}(m)} = \sqrt{\frac{{E_{k}^{ref}(m)} + {E_{k - 1}^{ref}(m)}}{{E_{k}(m)} + {E_{k - 1}(m)}}}}},} & (3)\end{matrix}$

[0032] where E^(ref) _(k)(m) is the reference energy, and E_(k)(m) isthe estimated energy, at the point m in time. This ensures that thegrouped channels get the same gain value. Such grouping of the gainfactors preserves the aliasing cancellation properties of the filterbankand gives the output according to FIG. 7. Here it is obvious that thealiasing components present in FIG. 3, are vanished. If there is nostrong sinusoidal component, the zeros will nevertheless be situated ineither half of the z-plane, indicated by the sign of the channel, andthe channels will be grouped accordingly. This means that there is noneed for detection based decision making whether there is a strong tonalcomponent present or not.

[0033] In a real-valued filterbank, the energy estimation is notstraightforward as in a complex representation. If the energy iscalculated by summing the squared subband samples of a single channel,there is a risk of tracking the time envelope of the signal rather thanthe actual energy. This is due to the fact that a sinusoidal componentcan have an arbitrary frequency from 0 to the filterbank channel width.If a sinusoidal component is present in a filterbank channel it can havea very low relative frequency, albeit being a high frequency sinusoidalin the original signal. Assessing the energy of this signal becomesdifficult in a real-valued system since, if the averaging time is badlychosen with respect to the frequency of the sinusoidal, a tremolo(amplitude-variation) can be introduced, when in fact the signal energyactually is constant.

[0034] The present invention teaches however, that the filterbankchannels should be grouped two-by-two given the location of thesinusoidal components. This significantly reduces the tremolo-problem,as will be outlined below.

[0035] In a cosine-modulated filterbank the analysis filters h_(k)(n)are cosine-modulated versions of a symmetric low-pass prototype filterp₀(n) as $\begin{matrix}{{h_{k}(n)} = {\sqrt{\frac{2}{M}}\quad {p_{0}(n)}\quad \cos \left\{ {\frac{\pi}{2M}\left( {{2k} + 1} \right)\left( {n - \frac{N}{2} - \frac{M}{2}} \right)} \right\}}} & (4)\end{matrix}$

[0036] where M is the number of channels, k=0, 1, . . . , M-1, N is theprototype filter order and n=0, 1, . . . , N. The symmetry of theprototype filter is assumed here to be with respect to n=N/2. Thederivations below are similar in case of half sample symmetry.

[0037] Given a sinusoidal input signal x(n)=A cos(Ωn+θ) with frequency0≦Ω≦π, the subband signal of channel k≦1 can be computed to beapproximately $\begin{matrix}{{{v_{k}(n)} \approx {\frac{A}{\sqrt{2M}}P\left\{ {\Omega - {\frac{\pi}{2M}\left( {{2k} + 1} \right)}} \right\} \cos \left\{ {{\Omega \quad {Mn}} + {\frac{\pi}{4}\left( {{2k} + 1} \right)} - \frac{N\quad \Omega}{2} + \theta} \right\}}},} & (5)\end{matrix}$

[0038] where P(ω) is the real valued discrete time Fourier transform ofthe shifted prototype filter p₀(n+N/2). The approximation is good whenP(Ω+π(k+½)/M) is small, and this holds in particular if P(ω) isnegligible for |ω|≧π/M, a hypothesis underlying the discussion whichfollows. For spectral envelope adjustment, the averaged energy within asubband k might be calculated as $\begin{matrix}{{{E_{k}(m)} = {\sum\limits_{n = 0}^{L - 1}\quad {{v_{k}\left( {{m\quad L} + n} \right)}^{2}{w(n)}}}},} & (6)\end{matrix}$

[0039] where w(n) is a window of length L. Inserting equation (5) inequation (6) leads to $\begin{matrix}{{{E_{k}(m)} = {\frac{A^{2}}{4M}P\left\{ {\Omega - {\frac{\pi}{2M}\left( {{2k} + 1} \right)}} \right\}^{2}\left\{ {{W(0)} + {{{W\left( {2\Omega \quad M} \right)}}{\cos \left( {{2\Omega \quad {MLm}} + {\frac{\pi}{2}\left( {{2k} + 1} \right)} + {\Psi (\Omega)}} \right)}}} \right\}}},} & (7)\end{matrix}$

[0040] where Ψ(Ω) is a phase term which is independent of k and W(ω) isthe discrete time Fourier transform of the window. This energy can behighly fluctuating if Ω is close to an integer multiple of π/M, althoughthe input signal is a stationary sinusoid. Artifacts of tremolo typewill appear in a system based on such single real analysis bank channelenergy estimates.

[0041] On the other hand, assuming that π(k−½)/M≦Ω≦π(k+½)/M and thatP(ω) is negligible for |ω|≧π/M, only the subband channels k and k−1 havenonzero outputs, and these channels will be grouped together as proposedby the present invention. The energy estimate based on these twochannels is $\begin{matrix}{{{{E_{k}(m)} + {E_{k - 1}(m)}} = {\frac{A^{2}}{4M}{S_{k}(\Omega)}\left\{ {{W(0)} + {{ɛ_{k}(\Omega)}{\cos \left( {{2\Omega \quad {MLm}} + {\frac{\pi}{2}\left( {{2k} + 1} \right)} + {\Psi (\Omega)}} \right)}}} \right\}}},} & (8)\end{matrix}$

[0042] where $\begin{matrix}\begin{matrix}{{S_{k}(\Omega)} = {{P\left\{ {\Omega - {\frac{\pi}{2M}\left( {{2k} + 1} \right)}} \right\}^{2}} + {P\left\{ {\Omega - {\frac{\pi}{2M}\left( {{2k} - 1} \right)}} \right\}^{2}}}} \\{and}\end{matrix} & (9) \\{{ɛ_{k}(\Omega)} = {{{W\left( {2\Omega \quad M} \right)}}{\frac{{P\left\{ {\Omega - {\frac{\pi}{2M}\left( {{2k} + 1} \right)}} \right\}^{2}} - {P\left\{ {\Omega - {\frac{\pi}{2M}\left( {{2k} - 1} \right)}} \right\}^{2}}}{S_{k}(\Omega)}.}}} & (10)\end{matrix}$

[0043] For most useful designs of prototype filters, it holds that S(Ω)is approximately constant in the frequency range given above.Furthermore, if the window w(n) has a low-pass filter character, then|ε(Ω)| is much smaller than |W(0)|, so the fluctuation of the energyestimate of equation (8) is significantly reduced compared to that ofequation (7).

[0044]FIG. 8 illustrates an inventive apparatus for spectral envelopeadjustment of a signal. The inventive apparatus includes a means 80 forproviding a plurality of subband signals. It is to be noted that asubband signal has associated therewith a channel number k indicating afrequency range covered by the subband signal. The subband signaloriginates from a channel filter having the channel number k in ananalysis filterbank. The analysis filterbank has a plurality of channelfilters, wherein the channel filter having the channel number k has acertain channel response which is overlapped with a channel response ofan adjacent channel filter having a lower channel number k−1. Theoverlapping takes place in a certain overlapping range. As to theoverlapping ranges, reference is made to FIGS. 1, 3, 4, and 7 showingoverlapping impulse responses number k−1. The overlapping takes place ina certain overlapping range. As to the overlapping ranges, reference ismade to FIGS. 1, 3, 4, and 7 showing overlapping impulse responses indashed lines of adjacent channel filters of an analysis filterbank.

[0045] The subband signals output by the means 80 from FIG. 8 are inputinto a means 82 for examining the subband signals as to aliasinggenerating signal components. In particular, the means 82 is operativeto examine the subband signal having associated therewith the channelnumber k and to examine an adjacent subband signal having associatedtherewith the channel number k−1. This is to determine whether thesubband signal and the adjacent subband signal have aliasing generatingsignal components in the overlapping range such as a sinusoidalcomponent as illustrated for example in FIG. 1. It is to be noted herethat the sinusoidal signal component for example in the subband signalhaving associated therewith channel number 15 is not positioned in theoverlapping range. The same is true for the sinusoidal signal componentin the subband signal having associated therewith the channel number 20.Regarding the other sinusoidal components shown in FIG. 1, it becomesclear that those are in overlapping ranges of corresponding adjacentsubband signals.

[0046] The means 82 for examining is operative to identify two adjacentsubband signals, which have an aliasing generating signal component inthe overlapping range. The means 82 is coupled to a means 84 forcalculating gain adjustment values for adjacent subband signals. Inparticular, the means 84 is operative to calculate the first gainadjustment value and a second gain adjustment value for the subbandsignal on the one hand and the adjacent subband signal on the otherhand. The calculation is performed in response to a positive result ofthe means for examining. In particular, the means for calculating isoperative to determine the first gain adjustment value and the secondgain adjustment value not independent on each other but dependent oneach other.

[0047] The means 84 outputs a first gain adjustment value and a secondgain adjustment value. It is to be noted at this point that, preferably,the first gain adjustment value and the second gain adjustment value areequal to each other in a preferred embodiment. In the case of modifyinggain adjustment values, which have been calculated for example in aspectral band replication encoder, the modified gain adjustment valuescorresponding to the original SBR gain adjustment values are bothsmaller than the higher value of the original values and higher than thelower value of the original values as will be outlined later on.

[0048] The means 84 for calculating gain adjustment values thereforecalculates two gain adjustment values for the adjacent subband signals.These gain adjustment values and the subband signals themselves aresupplied to a means 86 for gain adjusting the adjacent subband signalsusing the calculated gain adjustment values. Preferably, the gainadjustment performed by the means 86 is performed by a multiplication ofsubband samples by the gain adjustment values so that the gainadjustment values are gain adjustment factors. In other words, the gainadjustment of a subband signal having several subband samples isperformed by multiplying each subband sample from a subband by the gainadjustment factor, which has been calculated for the respective subband.Therefore, the fine structure of the subband signal is not touched bythe gain adjustment. In other words, the relative amplitude values ofthe subband samples are maintained, while the absolute amplitude valuesof the subband samples are changed by multiplying these samples by thegain adjustment value associated with the respective subband signal.

[0049] At the output of means 86, gain-adjusted subband signals areobtained. When these gain-adjusted subband signals are input into asynthesis filterbank, which is preferably a real-valued synthesisfilterbank, the output of the synthesis filterbank, i.e., thesynthesized output signal does not show significant aliasing componentsas has been described above with respect to FIG. 7.

[0050] It is to be noted here that a complete cancellation of aliasingcomponents can be obtained, when the gain values of the adjacent subbandsignals are made equal to each other. Nevertheless, at least a reductionof aliasing components can be obtained when the gain adjustment valuesfor the adjacent subband signals are calculated dependent on each other.This means that an improvement of the aliasing situation is alreadyobtained, when the gain adjustment values are not totally equal to eachother but are closer to each other compared to the case, in which noinventive steps have been taken.

[0051] Normally, the present invention is used in connection withspectral band replication (SBR) or high frequency reconstruction (HFR),which is described in detail in WO 98/57436 A2.

[0052] As it is known in the art, spectral envelope replication or highfrequency reconstruction includes certain steps at the encoder-side aswell as certain steps at the decoder-side.

[0053] In the encoder, an original signal having a full bandwidth isencoded by a source encoder. The source-encoder produces an outputsignal, i.e., an encoded version of the original signal, in which one ormore frequency bands that were included in the original signal are notincluded any more in the encoded version of the original signal.Normally, the encoded version of the original signal only includes a lowband of the original bandwidth. The high band of the original bandwidthof the original signal is not included in the encoded version of theoriginal signal. At the encoder-side, there is, in addition, a spectralenvelope analyser for analysing the spectral envelope of the originalsignal in the bands, which are missing in the encoded version of theoriginal signal. This missing band(s) is, for example, the high band.The spectral envelope analyser is operative to produce a coarse enveloperepresentation of the band, which is missing in the encoded version ofthe original signal. This coarse spectral envelope representation can begenerated in several ways. One way is to pass the respective frequencyportion of the original signal through an analysis filterbank so thatrespective subband signals for respective channels in the correspondingfrequency range are obtained and to calculate the energy of each subbandso that these energy values are the coarse spectral enveloperepresentation.

[0054] Another possibility is to conduct a Fourier analysis of themissing band and to calculate the energy of the missing frequency bandby calculating an average energy of the spectral coefficients in a groupsuch as a critical band, when audio signals are considered, using agrouping in accordance with the well-known Bark scale.

[0055] In this case, the coarse spectral envelope representationconsists of certain reference energy values, wherein one referenceenergy value is associated with a certain frequency band. The SBRencoder now multiplexes this coarse spectral envelope representationwith the encoded version of the original signal to form an outputsignal, which is transmitted to a receiver or an SBR-ready decoder.

[0056] The SBR-ready decoder is, as it is known in the art, operative toregenerate the missing frequency band by using a certain or allfrequency bands obtained by decoding the encoded version of the originalsignal to obtain a decoded version of the original signal. Naturally,the decoded version of the original signal also does not include themissing band. This missing band is now reconstructed using the bandsincluded in the original signal by spectral band replication. Inparticular, one or several bands in the decoded version of the originalsignal are selected and copied up to bands, which have to bereconstructed. Then, the fine structure of the copied up subband signalsor frequency/spectral coefficients are adjusted using gain adjustmentvalues, which are calculated using the actual energy of the subbandsignal, which has been copied up on the one hand, and using thereference energy which is extracted from the coarse spectral enveloperepresentation, which has been transmitted from the encoder to thedecoder. Normally, the gain adjustment factor is calculated bydetermining the quotient between the reference energy and the actualenergy and by taking the square root of this value.

[0057] This is the situation, which has been described before withrespect to FIG. 2. In particular, FIG. 2 shows such gain adjustmentvalues which have, for example, been determined by a gain adjustmentblock in a high frequency reconstruction or SBR-ready decoder.

[0058] The inventive device illustrated in FIG. 8 can be used forcompletely replacing a normal SBR-gain adjustment device or can be usedfor enhancing a prior art gain-adjustment device. In the firstpossibility, the gain-adjustment values are determined for adjacentsubband signals dependent on each other in case the adjacent subbandsignals have an aliasing problem. This means that, in the overlappingfilter responses of the filters from which the adjacent subband signalsoriginate, there were aliasing-generating signal components such as atonal signal component as has been discussed in connection with FIG. 1.In this case, the gain adjustment values are calculated by means of thereference energies transmitted from the SBR-ready encoder and by meansof an estimation for the energy of the copied-up subband signals, and inresponse to the means for examining the subband signals as to aliasinggenerating signal components.

[0059] In the other case, in which the inventive device is used forenhancing the operability of an existing SBR-ready decoder, the meansfor calculating gain adjustment values for adjacent subband signals canbe implemented such that it retrieves the gain adjustment values of twoadjacent subband signals, which have an aliasing problem. Since atypical SBR-ready encoder does not pay any attention to aliasingproblems, these gain adjustment values for these two adjacent subbandsignals are independent on each other. The inventive means forcalculating the gain adjustment values is operative to derive calculatedgain adjustment values for the adjacent subband signals based on the tworetrieved “original” gain adjustment values. This can be done in severalways. The first way is to make the second gain adjustment value equal tothe first gain adjustment value. The other possibility is to make thefirst gain adjustment value equal to the second gain adjustment value.The third possibility is to calculate the average of both original gainadjustment values and to use this average as the first calculated gainadjustment value and the second calculated envelope adjustment value.Another opportunity would be to select different or equal first andsecond calculated gain adjustment values, which are both lower than thehigher original gain adjustment value and which are both higher than thelower gain adjustment value of the two original gain adjustment values.When FIG. 2 and FIG. 6 are compared, it becomes clear that the first andthe second gain adjustment values for two adjacent subbands, which havebeen calculated dependent on each other, are both higher than theoriginal lower value and are both smaller than the original highervalue.

[0060] In accordance with another embodiment of the present invention,in which the SBR-ready encoder already performs the features ofproviding subband signals (block 80 of FIG. 8), examining the subbandsignals as to aliasing generating signal components (block 82 of FIG. 8)and calculating gain adjustment values for adjacent subband signals(block 84) are performed in a SBR-ready encoder, which does not do anygain adjusting operations. In this case, the means for calculating,illustrated by reference sign 84 in FIG. 8, is connected to a means foroutputting the first and the second calculated gain adjustment value fortransmittal to a decoder.

[0061] In this case, the decoder will receive an already“aliasing-reduced” coarse spectral envelope representation together withpreferably an indication that the aliasing-reducing grouping of adjacentsubband signals has already been conducted. Then, no modifications to anormal SBR-decoder are necessary, since the gain adjustment values arealready in good shape so that the synthesized signal will show noaliasing distortion.

[0062] In the following, certain implementations of the means 84 forproviding subband signals are described. In case the present inventionis implemented in a novel encoder, the means for providing a pluralityof subband signals is the analyser for analysing the missing frequencyband, i.e., the frequency band that is not included in the encodedversion of the original signal.

[0063] In case the present invention is implemented in a novel decoder,the means for providing a plurality of subband signals can be ananalysis filterbank for analysing the decoded version of the originalsignal combined with an SBR device for transposing the low band subbandsignals to high band subband channels. In case, however, the encodedversion of the original signal includes quantized and potentiallyentropy-encoded subband signals themselves, the means for providing doesnot include an analysis filterbank. In this case, the means forproviding is operative to extract entropy-decoded and requantizedsubband signals from the transmitted signal input to the decoder. Themeans for providing is further operative to transpose such low bandextracted subband signals in accordance with any of the knowntransposition rules to the high band as it is known in the art ofspectral band replication or high frequency reconstruction.

[0064]FIG. 9 shows the cooperation of the analysis filterbank (which canbe situated in the encoder or the decoder) and a synthesis filterbank90, which is situated in an SBR-decoder. The synthesis filterbank 90positioned in the decoder is operative to receive the gain-adjustedsubband signals to synthesize the high band signal, which is then, aftersynthesis, combined to the decoded version of the original signal toobtain a full-band decoded signal. Alternatively, the real valuedsynthesis filterbank can cover the whole original frequency band so thatthe low band channels of the synthesis filterbank 90 are supplied withthe subband signals representing the decoded version of the originalsignal, while the high band filter channels are supplied with the gainadjusted subband signals output by means 84 from FIG. 8.

[0065] As has been outlined earlier, the inventive calculation of gainadjustment values in dependence from each other allows to combine acomplex analysis filterbank and a real-valued synthesis filterbank or tocombine a real-valued analysis filter-bank and a real-valued synthesisfilterbank in particular for low cost decoder applications.

[0066]FIG. 10 illustrates a preferred embodiment of the means 82 forexamining the subband signals. As has been outlined before with respectto FIG. 5, the means 82 for examining from FIG. 8 includes a means 100for determining a low order predictor polynomial coefficient for asubband signal and an adjacent subband signal so that coefficients ofpredictor polynomials are obtained. Preferably, as has been outlinedwith respect to equation (1), the first predictor polynomial coefficientof a second order prediction polynomial as defined in the equation (1)is calculated. The means 100 is coupled to means 102 for determining asign of a coefficient for the adjacent subband signals. In accordancewith the preferred embodiment of the present invention, the means 102for determining is operative to calculate the equation (2) so that asubband signal and the adjacent subband signal are obtained. The signfor a subband signal obtained by means 102 depends, on the one hand, onthe sign of the predictor polynomial coefficient and, on the other hand,of the channel number or subband number k. The means 102 in FIG. 10 iscoupled to a means 104 for analysing the signs to determine adjacentsubband signals having aliasing-problematic components.

[0067] In particular, in accordance with the preferred embodiment of thepresent invention, the means 104 is operative to determine subbandsignals as subband signals having aliasing-generating signal components,in case the subband signal having the lower channel number has apositive sign and the subband signal having the higher channel numberhas a negative sign. When FIG. 5 is considered, it becomes clear thatthis situation arises for subband signals 16 and 17 so that the subbandsignals 16 and 17 are determined to be adjacent subband signals havingcoupled gain adjustment values. The same is true for subband signals 18and 19 or subband signals 21 and 22 or subband signals 23 and 24.

[0068] It is to be noted here that, alternatively, also anotherprediction polynomial, i.e., a prediction polynomial of third, forth orfifth order can be used, and that also another polynomial coefficientcan be used for determining the sign such as the second, third or forthorder prediction polynomial coefficient. The procedure shown withrespect to equations 1 and 2 is, however, preferred since it involves alow calculation overhead.

[0069]FIG. 11 shows a preferred implementation of the means forcalculating gain adjustment values for adjacent subband signals inaccordance with the preferred embodiment of the present invention. Inparticular, the means 84 from FIG. 8 includes a means 110 for providingan indication of a reference energy for adjacent subbands, a means 112for calculating estimated energies for the adjacent subbands and a means114 for determining first and second gain adjustment values. Preferably,the first gain adjustment value g_(k) and the second gain adjustmentvalue g_(k−1) are equal. Preferably, means 114 is operative to performequation (3) as shown above. It is to be noted here that normally, theindication on the reference energy for adjacent subbands is obtainedfrom an encoded signal output by a normal SBR encoder. In particular,the reference energies constitute the coarse spectral envelopeinformation as generated by a normal SBR-ready encoder.

[0070] Depending on the circumstances, the inventive method of spectralenvelope adjustment can be implemented in hardware or in software. Theimplementation can take place on a digital storage medium such as a diskor a CD having electronically readable control signals, which cancooperate with a programmable computer system so that the inventivemethod is carried out. Generally, the present invention, therefore, is acomputer program product having a program code stored on amachine-readable carrier, for performing the inventive method, when thecomputer-program product runs on a computer. In other words, theinvention is, therefore, also a computer program having a program codefor performing the inventive method, when the computer program runs on acomputer.

1. Apparatus for spectral envelope adjustment of a signal, comprising:means for providing a plurality of subband signals, a subband signalhaving associated therewith a channel number k indicating a frequencyrange covered by the subband signal, the subband signal originating froma channel filter having the channel number k in an analysis filterbankhaving a plurality of channel filters, wherein the channel filter havingthe channel number k has a channel response which is overlapped with achannel response of an adjacent channel filter having a channel numberk−1 in an overlapping range; means for examining the subband signalhaving associated therewith the channel number k and for examining anadjacent subband signal having associated therewith the channel numberk−1 to determine, whether the subband signal and the adjacent subbandsignal have aliasing generating signal components in the overlappingrange; means for calculating a first gain adjustment value and a secondgain adjustment value for the subband signal and the adjacent subbandsignal in response to a positive result of the means for examining,wherein the means for calculating is operative to determine the firstgain adjustment value and the second gain adjustment value dependent oneach other; and means for gain adjusting the subband signal and theadjacent subband signal using the first and the second gain adjustingvalues or for outputting the first and the second gain adjustment valuesfor transmission or storing.
 2. Apparatus in accordance with claim 1, inwhich the means for examining is operative to calculate signs of subbandsignals based on coefficients of prediction polynomials for the subbandsignal and the adjacent subband signal, and to indicate a positiveresult, when the signs have a predetermined relationship to each other.3. Apparatus in accordance with claim 2, in which the means forexamining is operative to apply an auto-correlation method or aco-variance method.
 4. Apparatus in accordance with claim 2, in whichthe prediction polynomial is a low order polynomial having a first ordercoefficient, wherein the order of the low order polynomial is smallerthan 4 and in which the means for examining is operative to use thefirst order coefficient for calculating the signs of the subbandsignals.
 5. Apparatus in accordance with claim 2, in which the means forexamining is operative to calculate the sign for a subband signal basedon the following equation: $\begin{matrix}{{{sign}(k)} = \left\{ {\begin{matrix}\left( {- 1} \right)^{k} & {{{if}\quad \alpha_{1}} < 0} \\\left( {- 1} \right)^{k + 1} & {{{if}\quad \alpha_{1}} \geq 0}\end{matrix},{0 < k < M},} \right.} & (2)\end{matrix}$

wherein k is the channel number, and α₁ is the first-order coefficient.6. Apparatus in accordance with claim 2, in which the predeterminedrelationship is defined such that the subband signal having associatedtherewith the channel number k has a first sign and the adjacent subbandsignal having associated therewith the channel number k−1 has a secondsign, which is opposite to the first sign.
 7. Apparatus in accordancewith claim 6, in which the first sign is negative, and the second signis positive.
 8. Apparatus in accordance with claim 1, in which the meansfor examining is operative to perform a tonal analysis for the subbandsignal and the adjacent subband signal for determining a tonal componenthaving a tonality measure above a tonality threshold.
 9. Apparatus inaccordance with claim 8, in which the means for examining is operativeto determine, whether the tonal component is in the overlapping range ofchannel k and channel k−1.
 10. Apparatus in accordance with claim 1,further comprising means for providing a first reference spectralenvelope value for the subband signal and a second reference spectralenvelope value for the adjacent subband signal, in which the means forcalculating is operative to determine a first energy measure indicatinga signal energy of the subband signal and a second energy measureindicating a signal energy of the adjacent subband signal, and in whichthe means for examining is further operative to calculate the first andthe second gain adjustment values based on a linear combination of thefirst reference spectral envelope value and the second referencespectral envelope value or a linear combination of the first energymeasure or the second energy measure.
 11. Apparatus in accordance withclaim 1, in which the means for calculating is operative to calculatethe first and the second gain adjustment values such that they differ byless than a predetermined threshold or are equal to each other. 12.Apparatus in accordance with claim 11, in which the predeterminedthreshold is lower than or equal to 6 dB.
 13. Apparatus in accordancewith claim 1, further comprising means for providing an unmodified firstgain adjustment value for the subband signal and an unmodified secondgain adjustment value for the adjacent subband signal, and in which themeans for calculating is operative to calculate the first and the secondgain adjustment values so that both are larger than or equal to a lowervalue of the first and the second unmodified gain adjustment values andsmaller or equal to a higher value of the first and the secondunmodified gain adjustment values.
 14. Apparatus in accordance withclaim 13, in which the unmodified first gain adjustment value and theunmodified second gain adjustment value are indicative for a spectralenvelope of an original signal in a frequency band, wherein thefrequency band is to be reconstructed by a spectral band replication.15. Apparatus in accordance with claim 1, further comprising a synthesisfilterbank for filtering the gain adjusted subband signals to obtain asynthesized output signal.
 16. Apparatus in accordance with claim 1, inwhich the analysis filterbank is a real valued filterbank, and in whichthe synthesis filterbank is a real valued filterbank.
 17. Apparatus inaccordance with claim 1, in which the analysis filterbank is acomplex-valued filterbank, and in which the synthesis filterbank is areal-valued filterbank.
 18. Apparatus in accordance with claim 1, inwhich the means for calculating is operative to calculate the first gainadjustment value and the second gain adjustment value based on anaverage energy of the subband signal and the adjacent subband signal.19. A method of spectral envelope adjustment of a signal, comprising:providing a plurality of subband signals, a subband signal havingassociated therewith a channel number k indicating the frequency rangecovered by the subband signal, the subband signal originating from achannel filter having the channel number k in an analysis filterbankhaving a plurality of channel filters, wherein the channel filter havingthe channel number k has a channel response which is overlapped with achannel response of an adjacent channel filter having a channel numberk−1 in an overlapping range; examining the subband signal havingassociated therewith the channel number k and for examining an adjacentsubband signal having associated therewith the channel number k−1 todetermine, whether the subband signal and the adjacent subband signalhave aliasing generating signal components in the overlapping range;calculating a first gain adjustment value and a second gain adjustmentvalue for the subband signal and the adjacent subband signal in responseto a positive result of the means for examining, wherein the means forcalculating is operative to determine the first gain adjustment valueand the second gain adjustment value dependent on each other; and gainadjusting the subband signal and the adjacent subband signal using thefirst and the second gain adjusting values or outputting the first andthe second gain adjustment values for transmission or storing. 20.Computer program having a program code for performing a method, when thecomputer program runs on a computer, the method having the followingsteps: providing a plurality of subband signals, a subband signal havingassociated therewith a channel number k indicating the frequency rangecovered by the subband signal, the subband signal originating from achannel filter having the channel number k in an analysis filterbankhaving a plurality of channel filters, wherein the channel filter havingthe channel number k has a channel response which is overlapped with achannel response of an adjacent channel filter having a channel numberk−1 in an overlapping range; examining the subband signal havingassociated therewith the channel number k and for examining an adjacentsubband signal having associated therewith the channel number k−1 todetermine, whether the subband signal and the adjacent subband signalhave aliasing generating signal components in the overlapping range;calculating a first gain adjustment value and a second gain adjustmentvalue for the subband signal and the adjacent subband signal in responseto a positive result of the means for examining, wherein the means forcalculating is operative to determine the first gain adjustment valueand the second gain adjustment value dependent on each other; and gainadjusting the subband signal and the adjacent subband signal using thefirst and the second gain adjusting values or outputting the first andthe second gain adjustment values for transmission or storing.
 21. Amethod for spectral envelope adjustment of a signal, using a filterbankwhere the filterbank comprises a real valued analysis part and a realvalued synthesis part or where said filterbank comprises a complexanalysis part and a real valued synthesis part, where a lower, infrequency, channel and an adjacent higher, in frequency, channel aremodified using the same gain value, if the lower channel has a positivesign and the higher channel has a negative sign, so that a relationbetween subband samples of the lower channel and subband samples of thehigher channel is maintained.